The present invention relates to an automated blood pressure measuring apparatus and method. More specifically, the present invention relates to an automated non-invasive blood pressure (NIBP) monitor that utilizes a continuous non-invasive blood pressure (CNIBP) monitor to enhance the performance of the NIBP monitor.
Automated blood pressure monitoring has rapidly become an accepted and, in many cases, essential aspect of human treatment. Such monitors are now a conventional part of the patient environment in emergency rooms, intensive and critical care units, and in the operating theater.
The oscillometric method of measuring blood pressure involves applying an inflatable cuff around an extremity of a patient's body, such as a patient's upper arm. The cuff is inflated to a pressure above the patient's systolic pressure and then the cuff pressure is reduced either continuously or incrementally in a series of small steps. A pressure sensor measures the cuff pressure, including the pressure fluctuations resulting from the beat-to-beat pressure changes in the artery under the cuff. The data from the pressure sensor is used to compute the patient's systolic pressure, mean arterial pressure (MAP) and diastolic pressure
An example of the oscillometric method of measuring blood pressure is shown and described in U.S. Pat. Nos. 4,360,029; 4,394,034; and 4,638,810, which are commonly assigned with the present invention.
Although NIBP methods, such as the oscillometric method described above, are effective in determining the blood pressure of a patient, it is frequently desired to be able to measure the blood pressure on a continuous basis, such as with a hospitalized patient. One technique of providing a continuously measured blood pressure is to insert a saline filled catheter through a patient's vascular system to the point at which the blood pressure measurements are desired. The catheter is connected to a pressure sensor, which measures the pressure in the vessel. As an alternative method, the catheter, rather than being fluid filled, could have a pressure sensor at the tip that directly senses the blood.
Although these catheter techniques have proven effective and continuously monitor a patient's blood pressure, both involve making an incision into the patient's skin for insertion of the catheter into the blood vessel. As a consequence, this invasive procedure entails some risk of complication to the patient and is in most cases undesirable.
Although several methods currently exist for providing a continuous, non-invasive blood pressure estimate, various factors can affect the accuracy of such measurements. For example, changes in the physiological state of the patient can bring about changes in the arterial wall elasticity. In general, changes in the arterial wall elasticity will affect the measured pulse wave velocity. If the elastic modulus of the arterial wall changes, the relationship between arterial pressure and arterial distension also changes. If the operating point obtained using a blood pressure cuff calibration is used, then any changes in the arterial elasticity will require re-calibration and, if no re-calibration is performed, errors in the pressure estimation can clearly occur.
U.S. Pat. No. 6,648,828, commonly assigned with the present application and incorporated herein by reference, teaches a method that utilizes pulse wave velocity to create a continuous, non-invasive blood pressure measurement. The '828 patent teaches a method of measuring the pulse wave velocity within a patient and a method of relating such pulse transit time to blood pressure. Although the '828 patent teaches a method of accurately measuring the PWV, the relationship of PWV to blood pressure can often be difficult to achieve.
Thus, although various CNIBP monitoring systems and methods currently exist, these systems provide a somewhat unreliable estimate and have not yet replaced NIBP monitoring systems.
During the use of a conventional NIBP monitoring system, the blood pressure cuff is placed around the arm of a patient and is inflated to a pressure that fully occludes the brachial artery to prevent blood flow. The cuff is then progressively deflated and a pressure transducer detects pressure pulses as blood begins to flow past the pressure cuff. As can be understood, the selection of the initial inflation pressure determines the amount of time and deflation required before the NIBP system begins to detect cuff oscillations and blood flow. If the initial inflation pressure is selected well above the systolic blood pressure for the patient, the NIBP system over inflates the blood pressure cuff, resulting in patient discomfort and extended measurement time. Alternatively, if the initial inflation pressure is selected below the systolic blood pressure for the patient, the blood pressure cuff must reinflate to obtain an accurate reading. Therefore, it is desirable to utilize the blood pressure estimate from a CNIBP system to enhance the performance of a NIBP system.